Type Hint the Python 3.8 LRU Cache Example

[adam-grant-hendry]

Can someone please provide me guidance as to the best (recommended) way to type hint the LRU cache example from the Python 3.8 docs. To reiterate, the example source is:

class LRU(OrderedDict):
    'Limit size, evicting the least recently looked-up key when full'

    def __init__(self, maxsize=128, /, *args, **kwds):
        self.maxsize = maxsize
        super().__init__(*args, **kwds)

    def __getitem__(self, key):
        value = super().__getitem__(key)
        self.move_to_end(key)
        return value

    def __setitem__(self, key, value):
        if key in self:
            self.move_to_end(key)
        super().__setitem__(key, value)
        if len(self) > self.maxsize:
            oldest = next(iter(self))
            del self[oldest]

I'd like to start with the __init__() method first.

I know I can type hint *args as suggested in PEP 646: *args as a Type Variable Tuple, but without a kind of variadic generic support for dictionaries, I'm not sure how to type **kwds. My thoughts are:

1. Any: I'd prefer not to use this as it essentially disables type checking

From the cpython source, I know OrderedDict uses **kwds in __init__(), so I can't manually specify each of the keyword arguments by name.

The LRU cache is also a kind of specialized container, so perhaps something like a Protocol and TypeVar can be used, but I don't know how to make that work for **kwds.

Any suggestions?

[JelleZijlstra]

I don't think PEP 646 is useful here. The *args/**kwargs are forwarded to the OrderedDict constructor, which appears to be the same as the dict constructor (according to typeshed). So a precise type signature would simply duplicate the dict.__init__ signature: https://github.com/python/typeshed/blob/8a47a28e768c577385d6232d4afea496b60403e7/stdlib/builtins.pyi#L1022.

[adam-grant-hendry]

I didn't see the dict.__init__ signature! Thank you! I will study the builtins.

[adam-grant-hendry]

@JelleZijlstra So is the best way to type hint the __init__ this:

from __future__ import annotations

from collections import OrderedDict
from typing import Iterable, TypeVar, overload

from _typeshed import SupportsKeysAndGetItem

_KT = TypeVar('_KT')
_VT = TypeVar('_VT')

class LRU(OrderedDict):

    @overload
    def __init__(self) -> None:
        ...

    @overload
    def __init__(self: dict[str, _VT], **kwargs: _VT) -> None:
        ...

    @overload
    def __init__(self, __map: SupportsKeysAndGetItem[_KT, _VT]) -> None:
        ...

    @overload
    def __init__(
        self: dict[str, _VT], __map: SupportsKeysAndGetItem[str, _VT], **kwargs: _VT
    ) -> None:
        ...

    @overload
    def __init__(self, __iterable: Iterable[tuple[_KT, _VT]]) -> None:
        ...

    @overload
    def __init__(
        self: dict[str, _VT], __iterable: Iterable[tuple[str, _VT]], **kwargs: _VT
    ) -> None:
        ...

    @overload
    def __init__(self: dict[str, str], __iterable: Iterable[list[str]]) -> None:
        ...

    def __init__(
        self: dict[str, _VT] | dict[str, str],
        __map_or_iterable: SupportsKeysAndGetItem[_KT, _VT]
        | SupportsKeysAndGetItem[str, _VT]
        | Iterable[tuple[_KT, _VT]]
        | Iterable[tuple[str, _VT]]
        | Iterable[list[str]]
        | None = None,
        **kwargs: _VT | None,
    ) -> None:
        # Put implementation here...
        pass

Oy vey that's a lot of code...